An air conditioner in which at least one outdoor unit and at least one indoor unit are mutually coupled by plural refrigerant pipes has been suggested. In the case where a temperature of an outdoor heat exchanger becomes equal to or less than 0° C. when this air conditioner performs a heating operation, the outdoor heat exchanger may be frosted. When the outdoor heat exchanger is frosted, ventilation to the outdoor heat exchanger is inhibited by the frost, and thus heat exchange efficiency in the outdoor heat exchanger may be degraded. Thus, when frosting occurs to the outdoor heat exchanger, a defrosting operation has to be performed to defrost the outdoor heat exchanger.
For example, in an air conditioner described in Patent Literature 1, an outdoor unit that includes a compressor, a four-way valve, an outdoor heat exchanger, and an outdoor fan is coupled to two indoor units, each of which includes an indoor heat exchanger, an indoor expansion valve, and an indoor fan, via a gas refrigerant pipe and a liquid refrigerant pipe. In the case where, in this air conditioner, a defrosting operation is performed during a heating operation, the rotation of the outdoor fan and the rotation of the indoor fan are stopped. In conjunction with this, the compressor is stopped once, the four-way valve is switched such that the outdoor heat exchanger is shifted from a state of functioning as an evaporator to a state of functioning as a condenser, and the compressor is activated again. When the outdoor heat exchanger functions as the condenser, a high-temperature refrigerant discharged from the compressor flows into the outdoor heat exchanger and melts frost formed on the outdoor heat exchanger. Thus, the outdoor heat exchanger can be defrosted.
By the way, when the defrosting operation is performed, a rotational speed of the compressor is preferably increased to be as high as possible. It is because, when the defrosting operation is performed by increasing the rotational speed of the compressor, an amount of the high-temperature refrigerant that is discharged from the compressor and flows into the outdoor heat exchanger is increased, a defrosting operation time is thus shortened, and the heating operation can be restored at an early stage. For this reason, the compressor is usually driven at a predetermined maximum value of the rotational speed (for example, 90 rps) during the defrosting operation.
However, there is a case where suction pressure of the compressor is reduced because condensation pressure is not increased by the heat exchange conducted between the frost and the refrigerant in the outdoor heat exchanger during the defrosting operation, and because of an ambient air temperature, an indoor temperature, a difference in height between an installation position of the outdoor unit and an installation position of the indoor unit, and the like during the defrosting operation. In the case where the compressor keeps driven at the above-described maximum value of the rotational speed when a degree of a reduction in the suction pressure is large, the suction pressure may be significantly reduced and fall below a performance lower limit value. Then, when the suction pressure falls below the performance lower limit value, the compressor may be damaged. In addition, such a problem that low-pressure protection control for stopping the compressor to prevent damage to the compressor is executed, a defrosting operation time is thus extended, and restoration of the heating operation is delayed is inherent.
In view of the above, such control that the suction pressure of the compressor is detected during the defrosting operation, and the rotational speed of the compressor is reduced at a time that the detected suction pressure becomes equal to or less than threshold pressure (for example, 0.1 Mpa) that is higher by a predetermined value than the performance lower limit value, so as to suppress the reduction in the suction pressure is executed. At this time, a predetermined minimum value (for example, 72 rps) is set as the rotational speed of the compressor, and the compressor is controlled such that the rotational speed thereof does not become the minimum value or lower.
During the defrosting operation, the rotational speed of the compressor is controlled within a control range that is defined by the maximum value (90 rps) and the minimum value (72 rps) of the rotational speed of the compressor described above in accordance with the suction pressure of the compressor detected during the defrosting operation. Accordingly, while the damage to the compressor and shifting to the low-pressure protection control are prevented, the defrosting operation is continuously performed. Thus, a delay in the restoration of the heating operation is prevented.
It should be noted that the above-described maximum value and minimum value of the rotational speed of the compressor are obtained in advance by a test or the like. The maximum value is the rotational speed of the compressor at which defrosting of the outdoor heat exchanger is completed as early as possible while the degree of the reduction in the suction pressure of the compressor is taken into consideration. Meanwhile, the minimum value is such a rotational speed that, if the rotational speed of the compressor is reduced therefrom, the defrosting operation is performed for a long time or the defrosting operation cannot be performed due to a significant reduction in a refrigerant circulation amount or stop of circulation of the refrigerant caused by a reduction in a pressure difference between discharge pressure and the suction pressure of the compressor.